Many inks, adhesives and other curable coatings comprise free radical based or cationic formulations which may be photo-cured by exposure to light, typically ultraviolet (UV) or short wavelength visible radiation. Applications include curing of large area coatings, adhesive curing, as well as the print processes such as inkjet printing. Curing uniformity is critical for many large area photo-induced curing processes.
For example, UV curable free radical based photo-reactive inks have increased in popularity for use in inkjet printers. Ink jet printers may be used to print on flexible substrates such as polyvinylchloride (PVC) and other flexible polymer materials, and rigid substrates such as metal, wood and plastics. Such inks are usually jetted on top of a substrate with one or more layers and pass under a UV or visible light source for curing. Photo-initiators in the ink formulation are activated by photons, e.g. UV light energy, to create free radicals, which are highly reactive with other components in the ink such as monomers and oligomers. The resulting free-radical initiated polymerization or cross-linking reaction results in a solidified ink layer. In a typical inkjet application, the irradiation period occurs in a fraction of a second or less. When the ink leaves the irradiation zone, the polymerization or solidification may continue, which is referred to as dark reaction. The dark reaction usually does not continue very long. Many people, therefore, consider the free radical polymerization reaction terminates instantly when it leaves the irradiation zone, comparing to the time scale of typical photo-polymerization experiments or typical UV curing processes. In the high speed ink jet printing applications, the dark reaction may, however, be comparable to or even longer than the traveling time between two spatially separated UV irradiation zones and/or the waiting time between adjacent exposures of the same UV source in multiple scanning mode. The polymerization reaction triggered by previous exposures may still be active during a subsequent UV exposure in a multiple UV exposure sequence in a UV ink jet printer printing process. Proper arrangement or adjustment of a UV system in a UV ink jet printer to utilize the dark reaction may allow for more optimized curing and result in a better print quality.
Typical parameters to assess a UV inkjet printer include print quality, print speed, print width, type of substrate, reliability, for example. Among these, the combination of print quality and speed is often considered most challenging. Beside the print heads, which controls how ink droplets are jetted, UV light sources used for curing play an important role in the influence of print quality and speed. Traditional UV light sources used in inkjet printers are typically mercury (Hg) arc lamps and another class of Hg lamp, a microwave or electrode-less bulb, although other gas discharge lamps may also be used. These lamps provide high enough power to cure most types of inks at print speeds used in the industry to date and are used in a wide range of printer systems. However, the amount of heat irradiated from gas discharge lamps is usually very high, which places constraints on system design. Overheating may cause operational and maintenance problems. Excessive heat also limits the ability of inkjet printers to print on some heat sensitive substrates. However, if the lamp power is lowered to avoid deleterious heating effects, there may be a trade off, e.g. in lower print quality and speed, or curing may not be achieved at all.
In recent years, solid state light emitting devices (LEDs), such as light emitting diodes, have been developed as alternative light sources for industrial processes such as photo-reactive or photo-initiated processes, e.g. photo-curing of inks, adhesives and other coatings. LEDs are more energy efficient than traditional gas discharge lamps. Solid state light sources may also be preferred for environmental reasons, as well as longer lifetime. UV LEDs have attracted a lot of attention because they generate less heat and consume less power than gas discharge lamps, for the same usable light output.
However even with the highest power UV LED chips available to date, inkjet printers that solely use UV LEDs for curing still have some problems such as low print quality and/or speed. Under some standard print quality examination tests, print samples produced by UV LED inkjet printers may show evidence of improper cure with surface curing problems, adhesion problem, or color bleeding problems. So there is a need to improve curing processes, for example for applications and processes where LEDs have replaced conventional UV gas discharge lamp light sources.
UV LED sources commonly used in the inkjet industry have LED lines packed close to each other so that jetted ink layers receives continuous irradiation. Many of the applications of UV LED sources in inkjet printers use bare LED chips, dies or arrays with direct illumination so that light is spread out or diffused. Examples of such arrangements are described in US Patent Publication no. US2007/0013757 by Mimaki and in U.S. Pat. No. 7,137,696 to CON-TROL-CURE. These arrangements may have difficulty in achieving an intensity that is high enough for good print quality for some applications. More densely packed LED chips may be provided to achieve high intensity; however liquid cooling may then be required which adds to system complexity and cost. Such UV LED heads are very expensive because of the density and large number of LED chips required.
Efforts to improve curing quality and speed have been focused primarily on providing light sources with higher beam intensities to deliver more power, requiring densely packed LEDs. For example, U.S. Pat. No. 7,470,921 to Summit discloses an apparatus comprising a UV LED device which provides an over focused beam, with a plurality of LEDs being arranged on a concave surface to provide a convergent or focused single beam. This type of focused beam may be overkill, i.e. delivering a high intensity over a short period of time may result in low curing efficiency. For reasons mentioned in copending U.S. patent application Ser. No. 12/582,492, “System, method, and adjustable lamp head assembly for ultra-fast UV curing”, while light intensity must be greater than a threshold to initiate photo-reactions, high intensity irradiation may exceed a saturation value, above which light is not utilized efficiently for photo reactions or photo curing.
Also as described therein, dark reactions or dark polymerization can contribute significantly to the final conversion. Thus, it may be preferred to having the ink layer irradiated by the first light beam, followed by a period for dark reaction, having the second UV irradiation by the second light beam, followed by dark reaction and so on so forth. In order to achieve highest curing efficiency, the period for dark reaction may be controlled through UV beam setting and adjusted to match ink chemistry and print speed.
For example, for scanning type inkjet printers with continuous irradiation, although the ink layers may receive multiple UV illuminations (i.e. multiple scans), the period between each illumination is determined e.g. by the configuration of the print engine and one or more light sources, and scanning rate, for the print process and usually does not provide the flexibility of adjustment to match the optimal UV irradiation requirements by the ink chemistry. Typically in known systems, one or two light sources are arranged adjacent to the print head, close enough to the print head to cure newly jetted ink once it is deposited on the substrate, but far enough so that stray light (or heat) does not initiate curing too soon, or adversely affect the ink before or during jetting. The period between each two illuminations may not effectively match the dark reaction requirements of the ink chemistry. In systems providing a focused single beam, such UV sources also do not take advantage of dark reactions effectively. These systems do not provide sufficient control of periods of irradiation vs. dark polymerization for optimizing or improving the cure efficiency.
U.S. patent application Ser. No. 12/582,492, discloses a system, method and lamp head assembly, which addresses some of above-mentioned problems, by providing for an adjustable beam profile, suitable for high speed printing. By allowing for adjustment of the beam profile, this solution provides for better matching of the illumination dependent on process parameters. However, for some applications this solution may not be suitable, or too complex, and alternative or simpler, lower cost solutions may be required.
Also even if the intensity and beam profile of a light source may be adjusted, it does not overcome the disadvantage mentioned above that in scanning type inkjet printers, the period between scans is fixed and dependent on the apparatus and cannot provide control over an interval of dark polymerization between periods of irradiation.
Thus known UV curing systems such as inkjet printers, and particularly scanning type inkjet printers, may not provide sufficient control of the spatial pattern of irradiation, and dark intervals, leading to problems with print quality or curing efficiency for some applications.